1. Field of the Invention
The present invention concerns a method of detecting variation in the diameter of a substantially cylindrical elongate optical body such as an optical fiber. The invention also provides an optical fiber fabrication method using a detection method of this kind. The fabrication method guarantees a regular optical fiber diameter with great accuracy.
2. Description of the Prior Art
The prior art discloses two methods of detecting variation in the diameter of an optical fiber.
Referring to FIG. 1, the two methods illuminate a portion of an optical fiber 1 which is assumed, for purposes of explanation, to lie on an axis N normal to a reference plane P. The incident illumination light beam F lies on an axis x in the reference plane P and is projected normally onto the fiber 1. The axis x is an oriented axis in the direction of propagation of the light beam. The optical fiber 1 moves relative to the light beam F along the normal axis N. As the result of optical propagation/reflection phenomena of the light in/on the fiber, a particular radiation pattern is produced around the fiber 1. The two prior art methods analyze this radiation pattern in a limited angular sector around the fiber by measuring intensity levels of light diffused by the portion of the optical fiber 1 at respective points in the reference plane that are delimited by said limited angular sector defined around the normal axis N. The angular sector in question is defined by two angles that have an apex formed by the intersection between the normal axis and the reference plane and that are oriented in a given circumferential direction, arbitrarily chosen as the clockwise direction, about the oriented axis x.
The first prior art method, described in the article by J. Daniere et al. entitled "Un nouvel appareillage pour enregistrer les fluctuations de diametre d'une fibre optique" ("A new device for recording optical fiber diameter fluctuations"), published in J. Phys. Sci. Intrum., vol. 18, 1985, delimits the acute angular sector for obtaining the radiation pattern by the two angles of 10.degree. C. and 80.degree. C. This angular sector is denoted S1 in FIG. 1. The luminosity levels at respective points in the sector are in practise measured by means of a CCD diode strip, for example. The radiation pattern obtained in the angular sector in question is in the form of a set of interference fringes which are the result of interference between light waves in the incident beam F that are reflected from the exterior surface of the fiber 1 on which said beam impinges and light waves refracted at the interior surface of the fiber on the side opposite said beam. A radiation pattern of this kind has the remarkable property of varying in accordance with the diameter of the fiber. To be more precise, variation in the diameter of the fiber causes angular translation of the pattern. It is therefore possible to detect variation in the diameter of the optical fiber by detecting modification of the angular radiation pattern. Modification of the radiation pattern can be detected with reference to a reference radiation pattern held in memory and obtained for an optical fiber having a reference diameter.
The second prior art method is implemented in exactly the same way as just described for the first method, except that the angular sector used to obtain the radiation pattern is delimited by the two angles of 170.degree. and 190.degree., as shown at S2 in FIG. 1. This method differs from the first method described in that the optical phenomena responsible for the effect obtained are not interference phenomena but rather backscattering phenomena. The effect obtained is exactly the same as that previously described in the sense that variation in the diameter of the optical fiber causes overall angular translation of the resulting radiation pattern in the 170.degree.-190.degree. acute angular sector.
Recent advances in fiber fabrication mean that the defects to be detected on an optical fiber are now in the order of 50 nm. The two prior art methods described above have the drawback that their sensitivity is too low to achieve this. For example, in the first method described the overall translation of the radiation pattern in the 10.degree.-70.degree. angular sector has a period of 0.45 .mu.m, which means that the radiation pattern in the 10.degree.-70.degree. angular sector is reproduced substantially identically for fiber diameters that are equal to within a multiple of a 0.45 .mu.m increment. Variation of the fiber diameter therefore moves the radiation pattern in translation relatively slowly. Limitations of optical detection and measuring devices are such that a fiber diameter variation in the order of 110 nm is the smallest that can be detected.
A first objective of the invention is to remedy the aforementioned drawback by providing a method of detecting variation in the diameter of an optical fiber with a sensitivity that is very high compared to the prior art technique.
A second objective of the invention is to provide an optical fiber fabrication method using a detection method of this kind.